tty: Correct tty buffer flush.

commit 64325a3be08d364a62ee8f84b2cf86934bc2544a upstream.

  The root of problem is carelessly zeroing pointer(in function __tty_buffer_flush()),
when another thread can use it. It can be cause of "NULL pointer dereference".
  Main idea of the patch, this is never free last (struct tty_buffer) in the active buffer.
Only flush the data for ldisc(buf->head->read = buf->head->commit).
At that moment driver can collect(write) data in buffer without conflict.
It is repeat behavior of flush_to_ldisc(), only without feeding data to ldisc.

Signed-off-by: Ilya Zykov <ilya@ilyx.ru>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>